They imagine old photos showing rows of machines enclosing patients ravaged by polio in 1950s wards - old-style technology tackling diseases rarely seen in this country today.

So it is a surprise to walk into a modern NHS hospital and find what is essentially a working antique keeping a patient alive.

John Prestwich's iron lung is a pleasant shade of cream and pastel green, and it can be found in St Thomas' Hospital on the banks of the Thames in London.

It remains a shocking sight - a cylinder the length of a family car, and probably weighing twice as much, with a seemingly disembodied human head emerging from one end.

It interrupts its quiet surroundings with a regular whoosh as an oversized bellows contraption at the rear pulls the air in and out of the casing.

Moving a little closer and peering through tiny submarine-style windows, you can see John's chest gently rise and fall in time.

The lung pulls air in and out of the lungs

John, paralysed from the chin down, is 64 - and holds the dubious distinction of being the longest surviving beneficiary of iron lung technology.

He has not taken a breath for himself since he was struck down with polio at the age of 17.

To him, the sound of the rising and falling bellows is the drumbeat of his life.

"This machine is like a comfortable friend, not a prison," he told BBC News Online.

"When you live with a noise like that all the time, you don't notice it at all. What is deafening is silence - if it stops working."

If this machine were to fail, without the assistance of the dedicated staff in the ward, he could be dead within three minutes.

To be fair, although John has needed artificial breathing for the past 48 years - and spent a decade-and-a-half encased in an iron lung during the 1950s and 1960s - this is not his daily routine.

75 years of the iron lung

First tested on humans in 1928 by American Philip Drinker

By 1939, there were almost 1,000 in use in the UK

Many patients paralysed by polio - first vaccine introduced in 1954

Modern technology has replaced this two-tonne monster with a chest-sized device shaped like a turtle-shell, and which does almost exactly the same thing, in the Hertfordshire home he shares with his wife Maggie.

However, this week, John has come down with a chest infection, and, since he cannot cough, must come into St Thomas' to spend a few days in the full-sized version while nurses pummel his chest to loosen the phlegm.

"It's no exaggeration to say that this unit, and the wonderful care I get here, has saved my life on many occasions," he said.

Polio pain

The idea behind the artificial lung is gloriously simple.

The typical patient cannot use his or her breathing muscles to inflate the lungs, but if a way could be found to draw up the chest, it would create a space in the lungs that would automatically be filled by air flowing in through the mouth and nose.

Once a patient is enclosed by the machine, a perfect seal is created. When the air is pumped out of the casing, the reduction in pressure makes the chest rise, filling the lungs.

This machine is decades old

When the air is allowed back in, the lungs empty.

The iron lung is forever linked in the mind with the devastating disease polio, and this is exactly how John became paralysed in 1955.

Polio attacks the nerves controlling the muscles, resulting in varying degrees of weakness, then paralysis.

Some are relatively lucky - they get away with impaired movement in an arm or a leg - but others, like John, are less fortunate.

The daily hard physical labour involved in his job as a deckhand on a merchant navy vessel meant that virtually all his muscles were affected.

This machine is like a comfortable friend, not a prison

John Prestwich

He woke up one evening and found he could not lift his face off his cabin pillow - which in itself, was nearly the end of him.

Medics resorted to desperate measures to save the life of the teenager as he headed for hospital.

"When I was taken off the boat, my breathing almost stopped completely.

"They had me laid on the stretcher, and were rocking me backwards and forwards, which moved air in and out of my lungs."

"When I woke up, I was a 17-year-old, surrounded by people I didn't know, I couldn't speak, there was something in front of me and this swishing noise.

"I never realised then that this noise would be with me every minute of every day for the next 48 years."

Many would have given up hope at this point, but perhaps one of the secrets of John's survival was his determination not to be bettered by his condition.

"The only thing I regret is that, when I was 18, I didn't go and put a bet on me going to get my old age pension - I could have got great odds on that.

"I have always considered my disability to be my enemy, and I have fought the restrictions it placed upon me."

This took the form of making the effort to get out of hospital every now and again - a risky business for someone so dependent on machinery.

On one trip out, he found himself in deep trouble - and miles from the safety of the ward and his iron lung.

"The trouble was there was so little equipment back then. If I left the hospital, I had to be 'hand-pumped' with this little set of bellows.

"On one occasion, the person doing this expanded the bellows too far and they broke. That was certainly a hairy moment.

"He pulled the rubber back over the box and we got the driver to get back to the hospital at top speed."

Life transformed

On other occasions his absent minded companions would be a problem.

"We used to go to the cinema - but if the film got a bit exciting my breathing tended to go to cock!"

It is the last decade and the increase in computing power that has transformed his life.

His home is now fully automated, with a computer which can respond to his whistled commands.

"When this first happened, I couldn't even ring the alarm bell - but now I can send a fax to America."

Some things, though, are likely to remain beyond him.

"I'm due for my free bus pass in a couple of weeks," he says. "Can't see me using it much."